The present invention relates to a method of controlling the solvent vapor concentration in an apparatus as well as apparatuses useful therefore. The solvent vapor is mixed with air and is obtained in the treatment of articles, such as a cleaning process, in the apparatus.
Methods and apparatuses for treating, for example cleaning, articles with organic solvents are well known. They are for example used in the dry cleaning of textiles etc. and for the degreasing of metals. The organic solvents are recovered during the drying phase in order to avoid losses of solvents and environmental pollution associated therewith. In some cleaning processes the gaseous organic solvents which are in the apparatus during the drying phase are cooled and condensed in a condenser. However, mainly hydrocarbons and particularly chlorinated and/or fluorinated hydrocarbons are used as solvents in the cleaning process. Many of them, for example, trichlorotrifluroethane, 1,1,1-trichloroethane, trichloromonofluoromethane, perchloroethylene, trichloroethylene or methylene chloride have a low boiling point and cannot be recovered by condensation to a sufficient extent. Therefore, the non-condensed organic solvents are usually adsorbed by adsorbents, such as activated carbon. Several processes have been suggested for recovering the organic solvents in the cleaning of articles.
U.S. Pat. No. 3,883,325 suggests recovering solvent vapors from an air stream which is recycled through an air cooler and then through an adsorbing unit in which the air stream, after it has left the cooler, is heated again before entering the adsorber. It is suggested that the air stream is first passed through the cooler bypassing the adsorber and then, after the solvent concentration in the air has been reduced, the adsorber is connected into the circuit. Activated carbon is used for adsorbing the organic solvents.
British patent specification 996,578 relates to a dry cleaning apparatus comprising a cleaning drum, a condenser and a solvent adsorber which is filled with activated carbon. German Offenlegungsschrift 37 37 783 also relates to a dry cleaning machine having a cleaning drum, a condenser and an adsorber containing activated carbon.
According to the teachings of U.S. Pat. No. 3,883,325, British patent 996,578 and German Offenlegungsschrift 37 37 783, activated carbon is regenerated with steam, the steam-solvent mixture is condensed and water is separated from the organic solvent. This necessary separation step is disadvantageous.
Accordingly, other methods and means for regenerating the adsorbent, such as activated carbon, have been suggested.
U.S. Pat. No. 4,583,985 relates to the recovery of organic solvents in dry cleaning and solvent finishing. In the taught process the solvents which occur in the form of a solvent vapor/air mixture are partially condensed by cooling. A saturation partial pressure of the solvent in the solvent vapor/air mixture of not more than 0.25 is set up. The solvent vapors are adsorbed with activated carbon while the solvent free air is blown out to the environment. The adsorbed solvents are desorbed by means of a solvent vapor/air mixture of the same concentration as in the adsorption process and at temperatures of 100.degree. C. to 150.degree. C., the flow through the activated carbon being in the opposite direction to that in the adsorption process. The desorbed solvent is condensed in a condenser. However, a condenser having a relatively high heat exchange area and a high performance compressor are necessary in combination with a liquefier in order to reach this saturation partial pressure. Furthermore, although a high amount of solvent vapor is adsorbed in the activated carbon, there is still a risk of solvent release to the environment. An undesirably high amount of solvent vapor may be released to the environment if the adsorption capacity of the activated carbon is close to exhaustion or is exhausted.
German Patent 36 09 578 (equivalent to U.S. Pat. No. 4,788,776) suggests a dry-cleaning machine containing a cleaning drum, a heat exchange unit, consisting of a cooler and a heater, one adsorber and a fan for circulating air. The conduit system has several by-passes which allow the following connections, seen in the direction of the circulating air: a) cleaning drum - heat exchange unit - cleaning drum, b) cleaning drum - adsorber - cleaning drum, c) adsorber - heat exchange unit - adsorber. In the disclosed cleaning process the solvent vapor is partially condensed by circulating the solvent vapor/air mixture in conduit system b) and the adsorbed solvent is then desorbed by circulating air in conduit system c) wherein the air is heated in the heater, the hot air desorbs the solvent in the adsorber at a high temperature and the desorbed solvent is condensed in the cooler. However, the necessary by-passes require considerable space and at least four butterfly valves.
For avoiding these by-passes and butterfly valves German Offenlegungsschrift 37 26 245 (equivalent to U.S. Pat. No. 4,850,119) suggests, after partial condensation of the solvent vapors originating from the cleaning drum, removal of a portion of the circulating air stream which portion is passed over an adsorber. Two dependent air circulations are suggested. The adsorbent in the adsorber is regenerated with hot gas and the desorbed solvent is condensed in a cooler. Unfortunately, the suggested process and equipment are relatively complicated.
European patent 157 090 suggests a cleaning machine containing a treating chamber, a drying compartment and a drying gas circuit. The cleaned articles are dried by means of a drying gas. In the drying gas circuit are arranged, seen in the direction of the circulating drying gas, a cooler, an adsorber and the drying compartment. In the drying and desorption phase of the cleaning process, the solvent vapor of the drying gas is partially condensed in the condenser and the drying gas is then passed over the heated adsorber for desorbing solvent vapor from the adsorber. The desorbed solvent vapor is condensed in the condenser. The drying gas is purified by passing it over a cooled adsorber. However, the condensation of all the solvent vapors is energy-consuming.
Due to the deficiencies of the known methods of recovering solvent vapors and of the known apparatuses for treating articles, it would be desirable to provide a simple solution for solving the problems relating to the solvent vapors in the treatment of articles, such as cleaning operations. Specifically, it would be desirable to avoid the contact of the solvent vapors with steam during their desorption. Furthermore, it would be desirable to provide a very simple apparatus for treating articles which minimizes the contamination of the environment with solvent vapors during its operation.